Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Flow transitions in vacuum arc remelting

Abstract

The metallurgical structure of an ingot produced by vacuum arc remelting (VAR) depends critically on the temperature distribution within the liquid portion of the partially solidified ingot. This, in turn, depends on the fluid motion in the pool, since the dominant mechanism for transporting heat is convection. There are three primary sources of motion: buoyancy; Lorentz forces arising from the passage of current through the pool; and Lorentz forces arising from the presence of external inductors. These forces are constantly in competition with each other, and each tends to induce a quite different distribution of velocity and temperature. We examine the transition between these different flow regimes and derive dimensionless criteria which determine which regime is dominant. We show that the structure of an ingot produced by VAR depends critically on the temperature distribution within the liquid portion of the partially solidified ingot. This, in turn, depends on the fluid motion in the pool, since the dominant mechanism for transporting heat is convection. There are three primary sources of motion: buoyancy; Lorentz forces arising from the passage of current through the pool; and Lorentz forces arising from the presence of external inductors. These forces are constantly in competition with each other, and each tends to induce a quite different distribution of velocity and temperature. We examine the transition between these different flow regimes and derive dimensionless criteria which determine which regime is dominant. We show that modest changes in ingot current can produce radical changes in temperature distribution, and that weak, steady magnetic fields, of only ～1 Gs, can induce a powerful swirling motion which suppresses the normal flow.     

Deformation and fracture behaviour of porous FVS0812 aluminium alloy prepared by spray deposition

Abstract

The deformation and fracture behaviour of a porous FVS0812 aluminium alloy prepared by spray deposition was investigated using compression testing. Rapid densification of the porous alloy was observed before a height reduction of ~50%. The optimum strain rate and deformation temperature to achieve maximum density were determined. Microstructural evolution involved the following stages: breakup of boundaries between adjacent particles; plastic deformation of particles; and collapse of large pores into various smaller pores and their disappearance. A strain hardening phenomenon occurred at all strains when compression was carried out at at 573 and 673 K, while a recovery mechanism came into operation at 773 K and a true strain of 0.2. The critical strain for the occurrence of strain softening in the porous alloy was much higher than that for the fully dense alloy. The fracture criteria curve of the porous alloy was a straight line, with a slope of ~0.45268 at high temperature and ~0.47636 at room temperature, almost parallel to that of the fully dense alloy in homogeneous compression.     

Heterogeneous deformation and recrystallisation of iron base oxide dispersion strengthened PM2000 alloy

Abstract

The recrystallisation behaviour of PM2000 oxide dispersion strengthened ferritic alloy has been investigated for tube specimens which had been cold deformed after extrusion. The evolution of the recrystallisation temperature, defined as the minimum temperature at which the specimen begins to recrystallise, has been studied in detail as a function of the level of deformation. The microstructure was characterised using optical and transmission electron microscopy, together with microhardness measurements, and local texture measurements obtained using the electron backscattering pattern technique. The results can be interpreted if it is assumed that any procedure that produces a heterogeneous microstructure, stimulates recrystallisation. In this sense, larger strain gradients lead to more refined and more isotropic grain structures. The way in which these results can be exploited for commercial applications is discussed.     

Very strong low temperature bainite

Abstract

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10^-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.     

Determination of potency factor of cobalt for estimation of nickel equivalent in 16Cr–2Ni martensitic stainless steel

Abstract

Cobalt is considered to be a potential substitute for nickel in 16Cr–2Ni martensitic stainless steel. However, the percentage of cobalt that can substitute 1% of nickel remains to be determined. A computer program was developed for accurate prediction of δ ferrite content in Schaeffler type diagrams. The δ ferrite content in the steels was measured by metallography and estimated using the computation technique for various Schaeffler type diagrams. The percentage error in the experimentally measured and computed δ ferrite contents was ±1·0%. From the present study, the potency factor of cobalt was computed to be 0·64 that of nickel in the estimation of nickel equivalent.     

Modelling correlation between hot working parameters and flow stress of IN625 alloy using neural network

Abstract

In this work, an optimum multilayer perceptron neural network is developed to model the correlation between hot working parameters (temperature, strain rate and strain) and flow stress of IN625 alloy. Three variations of standard back propagation algorithm (Broyden, Fletcher, Goldfarb and Shanno quasi-Newton, Levenberg–Marquardt and Bayesian) are applied to train the model. The results show that, in this case, the best performance, minimum error and shortest converging time are achieved by the Levenberg–Marquardt training algorithm. Comparing the predicted values and the experimental values reveals that a well trained network is capable of accurately calculating the flow stress of the alloy as a function of the processing parameters. Sensitivity analysis revealed that temperature has the largest effect on the flow stress of the alloy being in good agreement with the metallurgical fundamentals.     

Impression creep of Sn–40Pb–2·5Sb peritectic solder alloy

Abstract

The power law creep behaviour of the Sn–40Pb–2·5Sb peritectic solder alloy was investigated using an impression test apparatus. The tests were carried out under constant stress in the range 17 to 39 MPa and at temperatures in the range 296 to 363 K. Assuming a power law relationship between the impression velocity and stress, power law stress exponents in the range 1–3 were determined. Analysis of the data showed that for all loads and temperatures, the activation energy was stress independent with values in the range 51–56 kJ mol^-1. Based on the stress exponents obtained and activation energy data, it is proposed that grain boundary diffusion is the major mechanism for creep of the Sn–Pb–Sb peritectic alloy under these test conditions.     

Modelling hot deformation of Inconel 718 using state variables

Abstract

A dislocation density based state variable model has been developed to describe the characteristic flow stress behaviour during hot deformation of polycrystalline superalloy Inconel 718. Model equations have been formulated to describe the role of the evolving microstructures on the macroscopic flow stress response to deformation. Following a peak in the flow stress associated with strain hardening, the model utilises mechanisms associated with dynamic recovery and recrystallisation to explain the gradual decrease in flow stress with continued deformation. Incorporation of these microstructure based state variables also enables prediction of microstructures associated with a range of hot deformation conditions. Model flow stress predictions have been validated against isothermal uniaxial compression tests conducted over a range of temperatures and strain rates relevant to industrial forging conditions.     

Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

Diffusion bonding between Ti -6Al -4V alloy and microduplex stainless steel with copper interlayer

Abstract

In the present study, diffusion bonding was used to join Ti -6Al- 4V alloy to a microduplex stainless steel using a pure copper interlayer. The effects of heating rate and holding time on microstructural developments across the joint region were investigated. After bonding, microstructural analysis including metallographic examination and energy dispersive spectroscopy (EDS), microhardness measurements, and shear strength tests were carried out. From the results, it was seen that heating rate and holding time directly affect microstructural development at the joint, especially with respect to the formation of TiFe intermetallic compounds, and this in turn affects the shear strength of the bonds. A sound bond was obtained with a heating rate of 100 K min ^-1 and holding time of 5 min, and this was related to the small amount of TiFe intermetallics formed close to the duplex stainless steel side at this bonding condition. Although Ti₂Cu and TiFe intermetallics were formed in all specimens, it was seen that the most deleterious intermetallic was TiFe. As the heating rate was decreased and holding time increased the amount of TiFe intermetallics increased, and consequently shear strength decreased. As a result, from the microstructural observations, EDS analysis, microhardness measurements, and shear strength tests, it was concluded that a high heating rate and a short holding time must be used in the diffusion bonding of Ti-6Al- 4V to a microduplex stainless steel when pure copper interlayers are used.     

Refinement of Fe4Al13 in Al–Fe alloys by plasma remelting process

Abstract

Refinement of the Fe₄Al₁₃ phase in Al–Fe (wt-%) alloys has been achieved by a plasma remelting process. The refinement effect is enhanced by increases of discharge current and argon flux. The average size of the Fe₄Al₁₃ is 40 µm in Al–1Fe alloy melted using an electric resistance furnace. This can be reduced to 0.4 µm by a plasma remelting process. Similarly, the average size of the Fe₄Al₁₃ in Al–5Fe alloy can be decreased from 60 to 3 µm by plasma remelting. The refinement is considered to be an effect of the decrease of cluster size retaining genetic information in the Al–Fe alloy melt, the homogenisation of microstructure in the melt, and the increased undercooling during solidification.     

Response of titanium alloys to high strain rate deformation

Abstract

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10^?1 s^?1 to 10³ s^?1 and samples of Ti811 and Ti153 at a strain rate of 10³ s^?1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.     

Deformation texture and microtexture developments in a cold rolled single phase hexagonal Zircaloy 2

Abstract

A single phase hexagonal close packed Zircaloy 2 was cold deformed to different reductions by laboratory rolling. Systematic characterisations of the structural developments were carried out. Bulk texture developments were gradual, strongest developments being noticed at the highest strain. Although formation of well defined deformation fibre(s) could not be identified, overall developments in deformation texture were best captured through Taylor type models incorporating only prismatic slip. Strain localisations were observed as single or double walled dislocation structures at approximately 45 and 60° to the rolling direction. Such strain localisations were always associated with significant lattice reorientations or misorientation developments. Relative softening in lattice strain, observed at the higher reductions, can possibly be explained by the appearance of extensive strain localisations and the associated concurrent local dynamic recovery. The grains or orientations with dominant presence of strain localisations could be indirectly related to negative textural softening.     

Piezoresistive effect in amorphous carbon thin films

Abstract

In this contribution amorphous carbon (a-C) films are integrated as strain gauges in micromachined silicon boss membranes. Sputter deposited a-C films have high hardness and <2 % hydrogen content in it. The tribological properties of the a-C films are comparable with diamond and can be used for hard coatings. The films have very low resistivity which decreases with the temperature. Current voltage characteristics of a-C/oxide Si shows Ohmic behaviour. Variable range hopping mechanism is dominant at low temperatures and is thermally activated at room temperature and at higher temperatures. Piezoresistive gauge factor are measured in the temperature range 23–50°C.     

Stages of fatigue fracture in nickel alloyed powder metallurgy steel

Abstract

Fatigue fracture of smooth rectangular specimens was investigated by testing at constant amplitude and zero mean stress, at a frequency of 30 Hz. Two nickel alloyed powder metallurgy steels with different contents of pores and MnS were examined. Fatigue fracture started with crack nucleation at pore interfaces by the formation of small steps. Growth of these nuclei was accomplished by stepwise crack tip blunting, which led to the formation of macrocracks. A dominant fatigue crack origin built up at the region of the largest number of macrocracks. Fatigue crack growth stages I, II, and III were found in this region.     

Three-dimensional thermomechanical coupled FEM analysis of static recrystallisation during two-pass tube tension reducing process

Abstract

In this paper, a thermal dynamic numerical method was carried out to model the austenite static recrystallisation behaviour of steel 33Mn2V used in new non-quenched/tempered oil well tubes at different deformation temperatures, strain rates, deformation amounts and initial microstructures. Based on the MARC/AutoForge software, a three-dimensional thermomechanical coupled elastoplastic finite element model (FEM) was applied to simulate static recrystallisation amount of the two-pass tube tension reducing process of steel 33Mn2V for oil well. The distribution law of static recrystallisation inside the workpiece is analysed. It is indicated that the simulation results are much reliable through comparison with experimental data.     

Study of precipitation hardening in lead calcium tin anodes for copper electrowinning

Abstract

A suitable yield stress for Pb anodes with 0·07%Ca and 1·3%Sn of 6 mm thickness for copper electrowinning is achieved by means of deformation and precipitation hardening processes, their useful life being dependent on this yield stress. The objective of the present work was to optimise the precipitation hardening, by finding the best cooling conditions for the anodes in the moulds and the optimum hot rolling temperature. The results show that increasing the cooling rate of ingots beyond the rate of natural cooling enhances the precipitation hardening. At least 45 days of aging are necessary to reach stable conditions for the precipitation hardening, with precipitates formation as CaSn₃. The hot rolling temperature does not have a significant effect on the precipitation hardening of the anodes.     

Modelling steady state deformation of fcc metals by non-equilibrium thermodynamics

Abstract

The steady state of plastic deformation is modelled by non-equilibrium thermodynamics theory. Based on energy conservation and constant entropy requirements at the steady state, the saturation dislocation density ρ is found to be determined by ρ=λ ?/(bv _c), where λ is a constant that depends on the material properties, ? is the strain rate, b is the magnitude of Burgers vector and v _c is the dislocation climb velocity along the dislocation line. Then, by employing the Taylor relation, the saturation flow stress is obtained. The model is applied to four pure fcc single crystals under tensile testing and polycrystalline Al at steady state creep. The predictions are in good agreement with the experimental observations.     

Deformation behaviour and new constitutive equation utilising the grain size of commercial TC6 titanium alloy

Abstract

Isothermal compression tests on a commercial TC6 titanium alloy have been conducted at deformation temperatures of about 800 – 1040°C, strain rates of 0.001 – 50 s^-1 and height reductions of 30 – 50%. The microstructural evolution is represented through the measured grain size of the prior α-phase. Meanwhile, a new constitutive equation, which includes the grain size, is established for high temperature deformation behaviour. The procedure required to formulate a constitutive equation from the experimental results is presented. The constitutive equation to model the behaviour of the TC6 titanium alloy during high temperature deformation is validated and its formulation is presented. The results show that the present equation is satisfactory for describing the behaviour of the TC6 titanium alloy during high temperature deformation. The maximum difference between the calculated and the experimental results is less than 15%.